1. Field of the Invention
The present invention relates to a sheet feeding apparatus for feeding a sheet to an image forming apparatus and the like.
2. Related Background Art
Among conventional sheet feeding apparatuses used in image forming apparatuses such as printers, copying machines, facsimiles and the like, there is a sheet feeding apparatus of inclined surface separation type for effecting sheet separation by using an inclined surface. In the sheet feeding apparatus of inclined surface separation type, an inclined surface is provided in front of stacked sheets, and, when a sheet feeding roller abuts against an uppermost sheet in the sheet stack and feeds the sheets, the fed sheets abut against the inclined surface to be separated one by one.
An example of such a sheet feeding apparatus of inclined surface separation type will be explained with reference to FIGS. 14 to 16.
In FIG. 14, the sheet feeding apparatus 50 for successively feeding the stacked sheets comprises a sheet feeding tray portion 51 for stacking the sheets to be fed, a sheet feeding portion 52 for separating and feeding the stacked sheets one by one, and a conveying portion 53 for conveying the fed sheet.
The sheet feeding portion 52 includes a sheet feeding roller 54 rotated integrally with a gear (not shown), an idler gear 55 meshed with the gear (not shown) of the sheet feeding roller 54, an idler gear 56 meshed with the idler gear 55, a drive gear 57 meshed with the idler gear 56, a drive shaft 58 attached to be rotated together with the drive gear 57, and a sheet feeding roller holder 59 rotatably attached to the drive shaft 58 and adapted to rotatably support the sheet feeding roller 54, idler gears 55, 56 and drive gear 57.
The conveying portion 53 includes a lower guide 60 for guiding a lower surface of the sheet fed from the sheet feeding portion 52, an upper guide 61 for guiding an upper surface of the fed sheet, a conveying roller 62 for conveying the sheet guided by the lower and upper guides 60, 61 to an image forming portion, a rotatable conveying sub-roller 63 opposed to the conveying roller 62, a sub-roller holder 64 rotatably supported by the upper guide 61 and adapted to rotatably support the conveying sub-roller 63, and a conveying spring 65 for biasing the sub-roller holder 64 to urge the conveying sub-roller 63 against the conveying roller 62.
The sheet feeding tray portion 51 includes a sheet feeding tray 66 for supporting the sheets S in an inclined condition, and a separation inclined surface 67 contiguous to the sheet feeding tray 66 and against which leading ends of the stacked sheets abut and are held and which is adapted to separate the sheets one by one.
In the sheet feeding apparatus 50 having the above-mentioned arrangement, a sheet feeding operation is effected as follows.
The drive shaft 58 receives a driving force from a controllable driving mechanism (not shown), and start and stop of the sheet feeding operation are controlled. When the sheet feeding operation is started, the drive shaft 58 is rotated by the driving mechanism (not shown). Such rotation is transmitted to the sheet feeding roller 54 through the drive gear 57 and idler gears 56, 55, thereby rotating the sheet feeding roller 54.
A sheet feeding roller arm 59 for holding the sheet feeding roller 54 for rotation around the drive shaft 58 is subjected to a force by biasing means (not shown) or by its own weight to be rotated in an anti-clockwise direction, and, by this force, the sheet feeding roller 54 is slightly contacted with an uppermost sheet in the sheet stack rested on the sheet feeding tray 66 of the sheet feeding tray portion 51.
Accordingly, when the rotation of the sheet feeding roller 54 is started, a feeding force F due to a friction force of the sheet feeding roller 54 acts on the sheet stack S. The sheet stack is subjected to a reaction force F from the separation inclined surface 67, and, by this reaction force F, the sheet stack is bent along the separation inclined surface 67, and the sheet stack is advanced in the bent condition while abutting the leading end of the sheet stack against the separation inclined surface 67. In this case, if the plural sheets are fed out, only the uppermost sheet is separated and fed by the separation inclined surface 67.
The separated sheet is directed by the upper and lower guides 61, 60 and enters into a nip defined between the conveying roller 62 and the conveying sub-roller 63 biased toward the conveying roller 62 by the conveying spring 65 and is further conveyed by the conveying roller 62 toward a downstream direction.
In this way, in this sheet feeding apparatus of inclined surface separation type, the sheets can be separated and fed one by one by the separation inclined surface 67.
In such a sheet feeding apparatus of inclined surface separation type, the sheet stacking surface for stacking the sheets is fixed. Since an arrangement which is generally used and in which an intermediate plate for stacking sheets is rockably provided and is biased by a spring is not required, and, since additional separation claws or separation pads are not required because only the inclined surface may be provided as separation means, the construction becomes simpler and cheaper. Further, since thick sheets which could not separated by the separation claws can be separated, the kind of sheets to be separated is increased.
Incidentally, in the above-mentioned conventional technique, while an example that the inclined surface separation system is applied to the sheet feeding apparatus having the inclined sheet feeding tray was explained, the inclined surface separation system may be applied to a sheet feeding apparatus in which sheets are supported and fed out in a horizontal condition. Further, the separation inclined surface may be provided on a so-called sheet feeding cassette for stacking sheets detachably attachable to a main body of the apparatus.
An example that the separation inclined surface is provided on the sheet feeding cassette will be explained with reference to FIG. 17.
A sheet feeding cassette 71 detachably attachable to a main body 70 of the apparatus is mounted to the main body 70 of the apparatus from a direction shown by the arrow. The sheet feeding cassette 71 includes a resting surface 72 on which sheets S are stacked, a separation inclined surface 73 for separating the sheets, and a trailing end regulating plate 74 for regulating trailing ends of the sheets S stacked on the resting surface 72. The separation inclined surface 73 is located at a front part of the sheet feeding cassette 71 in a mounting direction.
The main body 70 of the apparatus includes a sheet feeding roller 75 for feeding out the sheets S contained in the sheet feeding cassette 71, and the sheet feeding roller 75 is supported by a sheet feeding roller holder 77 rotatable around a rotary fulcrum 76.
When the sheet feeding cassette 71 containing the sheets is mounted to the main body 70 of the apparatus and the sheet feeding roller 75 is rotated while contacting with the upper surface of the sheet stack, the sheets are fed, and the fed sheets abut against the separation inclined surface 73 and are separated one by one.
However, the above-mentioned conventional sheet feeding apparatus 50 arose the following problems.
When the sheet bundle (stack) is inserted onto the sheet feeding tray portion 51 forcibly for sheet replenishment, since the separation inclined surface 67 is inclined with respect to the sheet inserting direction, if the sheet stack abuts against the separation inclined surface 67 strongly, the leading end of the sheet stack will be deformed as shown in FIG. 15. Such deformation may cause double-feeding or multi-feeding when the sheets are fed out by the sheet feeding roller 54. Further, if a large number of sheets are inserted onto the sheet feeding tray 51 at once, due to deformation of some of sheets, the sheets are not stopped by the separation inclined surface 67 but are further slipped, with the result that the inserted sheets cannot be stacked at a predetermined position, while may result in the double-feeding.
Further, when the sheets are inserted into the sheet feeding tray portion 51 for sheet replenishment, since the sheet feeding roller 54 remains in a lowered condition as shown in FIG. 16, the leading end of the sheet stack abuts against the sheet feeding roller 54. In this condition, the sheets are hard to be inserted into the sheet feeding tray portion 51 and the sheet feeding roller 54 may be damaged by the leading end of the sheet stack. Further, when the sheets stacked on the sheet feeding tray portion 51 are removed for changing sheet size or sheet material, if the sheet feeding roller 54 is in the lowered condition to abut against the sheet stack, the sheets are hard to be removed from the sheet feeding tray portion 51 or cannot be removed, depending upon arrangement.
Further, in the sheet feeding apparatus as shown in FIG. 17 in which the separation inclined surface 73 is provided on the sheet feeding cassette 71, the following problem arose.
When the sheet feeding cassette 71 is dismounted from the main body 70 of the apparatus for sheet replenishment and then the sheet feeding cassette 71 is inserted into the main body 70 of the apparatus again after the sheet replenishment, if the cassette is inserted forcibly, the stacked sheets S strongly abut against the separation inclined surface 73 due to inertia. In this case, since the separation inclined surface 73 is inclined with respect to the inserting direction of the sheet feeding cassette 71, the leading end of the sheet stack S is deformed as shown in FIG. 17. Such deformation may cause the double-feeding.